High power, long life and high fuel efficiency are recently required for internal engines such as automobile engines, needing the reduction of friction loss in sliding portions. Thus, hard, amorphous carbon coatings having low friction coefficients are proposed.
Among them, a plasma CVD method and a reactive sputtering method using plasma formed by exciting reactive hydrocarbon gases such as methane, acetylene, etc. introduced into a vacuum chamber, by high-frequency discharge, direct current discharge, etc., can form hydrogen-containing, hard, amorphous carbon coatings having relatively smooth surfaces.
Such hydrogen-containing, hard, amorphous carbon coatings formed on sliding members such as engine parts should indispensably have high wear resistance. Many proposals have been made therefor. For example, JP 2006-283134 A discloses the addition of Si to a hydrogen-containing, hard, amorphous carbon coating, JP 2001-316800 A discloses a method for forming a metal-containing, hard, amorphous carbon coating, and JP 2001-261318 A discloses a method for adding graphite clusters to a hard, amorphous carbon coating.
In hydrogen-containing hard carbon coatings, part of carbon bonds constituting the coatings are terminated by hydrogen regardless of the production method. Accordingly, the hydrogen-containing hard carbon coatings fail to have as strong carbon bonds as those in hard, amorphous carbon coatings containing no hydrogen, suffering faster wearing when formed on sliding members used under a high load. This appears to be due to the mechanism that weak carbon bonds in the hard, amorphous carbon coating are broken by a sliding load, resulting in worn powder detached from the coating.
In the hard, amorphous carbon coating containing graphite clusters, Van der Waals bonds of graphite contained are weaker than sp2 and sp3 bonds of carbon atoms constituting the matrix, reducing the wear resistance of the coating. Even coatings to which metal elements are added have lower hardness than that of hard, amorphous carbon coatings containing no metal elements, indicating that the metal elements do not have a high function of improving wear resistance.
Unlike other metal elements, Si forms a stronger covalent bond to carbon, resulting in improved hardness. However, because only sp3 bond is used, some Si atoms are not fully bonded, resulting in brittle coatings, which are easily cracked.